The phrase "nom. prov." is an abbreviation for the term nomem provisorium which means, "a provisional name or a name that may be changed in the future." An example of this would be Psilocybe montanta (nom. prov.) which will soon officially be changed to the scientific name Deconica montana. It is only properly used when we know for a fact that the present name of the species will be changed in the scientific literature.
